Integrated capacitors and in particular trench capacitors, are of increasing importance in several fields of application. One such field is integrated circuits contained in portable wireless communication devices, such as mobile telephone handsets, where trench capacitors are proving useful, as a part of passive integration technology.
Mobile telephone handsets have witnessed a strong innovative drive to smaller and cheaper products with increased functionality and performance while still meeting the tight constraints for mass production within a short product lifetime. This combination of conflicting trends provides a motivation for the innovative improvement in the passive component content of a telephone.
It is estimated that in a single-mode telephone, passive components account for 90 percent of the component count, 80 percent of the size and 70 percent of the cost. High quality passive components are especially prevalent in the RF front end and radio transceiver sections of the telephone and thus tend to increase proportionally as the number of operating modes (and thus frequency bands) increase.
One direct approach to reducing the passive component content in a telephone is passive integration technology. This technique allows several passive components to be integrated, either into a substrate or as a stand-alone component.
Significant progress has already been demonstrated in technology partitioning within an RF module platform to optimize size, cost and performance, and this partitioning concept can be extended beyond integrated (L,C,R), circuits. For example, as shown in FIG. 1, decoupling, filtering and switching are all electrical functions which cannot be effectively integrated on active silicon and which are required in the generic circuit blocks which form the RF front end of a radio transceiver.
FIG. 1 shows the RF front end of a telephone handset, comprising a transceiver ASIC (application specific integrated circuit) 1, which is connected to a dual-band voltage controlled oscillator (VCO) 2, and thence via a dual-band power amplifier (PA) 3 and switchplexer 4 to the antenna 5. Receiver filters 6 are connected between the switchplexer and transceiver ASIC. As well as switching element 7, the switchplexer includes a decoupling capacitor 8, and RF (L,C) coupler 9. Further decoupling capacitors 8, and RF (L,C,R) circuits also comprise parts of the transceiver ASIC, VCO, an PA.
From FIG. 1 it is apparent that the passive components, and in particular decoupling capacitors, comprise a significant part of the component count, and thence the space and cost of the complete unit.
Passive integration into partitioned circuit elements and their combination within an RF module platform provides a unique and powerful tool for minimizing the passive component count within a cellular telephone and is a key enabler for developing smaller, cheaper and more powerful wireless products with lower loss and form factors suitable for mass production.
A key element in passive integration is the (integrated) capacitor technology. Existing technologies to produce integrated passive components include planar MIM (metal-insulator-metal) technology and trench capacitor technology, both of which are now in mass production, and RF modules containing both technologies are commercially available from, for example, NXP Semiconductors.
The planar MIM capacitor technology uses high-k ferroelectric thin films such as barium strontium titanate Ba1-xSrxTiO3— (BST), lead zirconate titanate PbZrxTi1-xO3 (PZT), or lead lanthanum zirconate titanate (PLZT) to achieve high capacitance densities. The high-k ferroelectric materials can offer relative permittivity ranging from several hundreds to more than thousand. The breakdown voltages are typically of 50-250 V.
The trench capacitor technology uses porous silicon to achieve high capacitance densities. The trench/pore arrays in macro-porous Si substrate have been demonstrated to enlarge the capacitor surface area of the order of 20 to 30 times. Dielectrics like SiO2 and Si3N4 with relative permittivity of 4-7 are used because they can be easily grown into the deep trenches. The breakdown fields are typically of 6-10 MV/cm, translating in breakdown voltages of typically 15-32 V.
However, the dielectric materials which have been available for use in the trench capacitor technology have lower relative permittivity than those achievable with planar MIM technology.
US patent application US2007/0040204 discloses a three-dimensional capacitor utilising BST or PZT, by providing a laminated foil on top of the semiconductor substrate, and forming high-k films within trenches within the laminated foil.
Thus there remains an ongoing need to provide an effective, integrated capacitor technology, which is cost-effective space efficient and allows use of high-k materials.